I downloaded it ages ago though a dial-up connection and I think it took over 3 hours, which is'nt too bad. I've finished BG with DSOTSC installed twice but unfortunately I never finished DSOTSC itself. Some triggers for certain quests were never set off for whatever reason, preventing me from completing them.
I still reccommend you DL it though. Some of the new items are great fun to use (if INCREDIBLY overpowered) so you should get to experience a decent amount of DSOTSC even without completing the missions.
SPOILER TIP:
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Kill the group of bounty hunters south of Gullykin ASAP. The items they leave are incredible. Just make sure you have a Dwarven fighter in your group

(Kaigan?)